In a typical projection display apparatus, the picture produced by an imager illuminated by an illumination system is projected on a viewing screen thanks to an imaging system.
It is naturally desirable for the user that the apparatus has a depth perpendicular to the screen as small as possible (slim projector). However, as the illumination and imaging systems are preferably located at one side of the apparatus (generally the bottom side), the small depth necessarily implies that light is incoming onto the screen with an angle varying over a wide range.
As the transmission factor of the screen depends on the incoming angle of light, as in any optical system, this results in an uncontrolled heterogeneous brightness level over the screen (generally the bottom part is brighter).